Measuring programmers is a stupid idea, but there're lot of checkpoints, which can be used as indicator. These indicators requires programming experience, e.g. if you wanna know that a solution is a long-term safe solution or just a quick hack, you should peek inside the code.
Fortunatelly, there's a sharp line between amateur and professional programmers, which can be judged by non-programmers (also, it requires some technical knowledge): the documentation. Amateurs don't make documentation, professionals do. The quality of the document may vary, but if there's no any, it's the sign of amateurism.